1. Field of the Invention
The present invention relates to a polarization compensator and a wavelength division multiplexing (WDM) apparatus, and in particular to a polarization compensator for compensating a polarization state of a lightwave and a wavelength division multiplexing apparatus using same.
Characteristics of optical phenomena differ, in many cases, depending on a polarization state such as a reflection and a refraction. Also, various optical circuits utilizing a polarization dependency have been considered in which a change of an electric quantity, a magnetic quantity, distortion, or the like is converted into a change of a refractive index. In such optical circuits, it is important to input a lightwave in a predetermined polarization state.
Also, in an optical transmission, a wavelength division multiplexing method has been adopted responsive to a recent demand for a large-capacity transmission. In this wavelength division multiplexing method, it is important to transmit many lightwaves whose wavelengths are different from each other through an optical fiber.
2. Description of the Related Art
[1] FIGS. 7A and 7B show an optical isolator 90 as an example of an optical circuit having a general polarization dependency. The optical isolator 90 is composed of a polarizer 91, a magneto-optical rotator 92 with 45xc2x0 taking advantage of the Faraday effect, and an analyzer 93 arranged on z-axis in series. The directions of the polarizer 91 and the analyzer 93 are respectively set to have 0xc2x0 and 45xc2x0 with respect to x-axis.
As shown in FIG. 7A, a lightwave 64 of a liner polarization, inputted from the polarizer 91, whose polarization plane P1 is on xz plane, is rotated by 45xc2x0 at the rotator 92 to pass through the analyzer 93 as an output lightwave 65 without losses. Oppositely, as shown in FIG. 7B, a polarization inputted from the analyzer 93 and rotated by 45xc2x0 is further rotated by 45xc2x0 at the rotator 92, and a polarization plane P2 becomes a linear polarization on yz plane, so that it can not pass through the polarizer 91.
Thus, the optical isolator 90 has a function of blocking that the linear polarization outputted from e.g. the analyzer 93 is reflected by e.g. an optical fiber to be passed through in the opposite direction.
Also, in order to pass through the optical isolator 90 without losses, the lightwave 64 must be a linear polarization whose polarization plane P1 is on the xz plane. Accordingly, in order for a lightwave of an arbitrary polarization to pass through the optical isolator 90, the polarization plane P1 is required to be converted into a linear polarization on the xz plane.
[2] FIG. 8 shows an arrangement of a wavelength division multiplexing (hereinafter, occasionally abbreviated as WDM) optical transmission apparatus 201. Lightwaves whose wavelengths are different from each other transmitted from optical transmitters 30_1-30_n (hereinafter, occasionally represented by a reference numeral 30) are respectively inputted to a coupler 35 through optical fibers 32_1-32xe2x80x94n (hereinafter, occasionally represented by a reference numeral 32), optical amplifiers 31_1-31_n (hereinafter, occasionally represented by a reference numeral 31), and optical fibers 32xe2x80x2_1-32xe2x80x2_n (hereinafter, occasionally represented by a reference numeral 32xe2x80x2).
The coupler 35 couples the inputted lightwaves to be outputted to an optical fiber 36. An optical amplifier 38 amplifies the lightwaves to which the wavelength division multiplexing is performed, and relays the same to the subsequent stage. Thus, a wavelength division multiplexing is a technology capable of increasing a transmission capacity of a single optical fiber by transmitting many lightwaves whose optical frequencies are different from each other through the single optical fiber.
For increasing the number of wavelengths in the wavelength division multiplexing, it is effective to narrow an interval between optical frequencies whose wavelengths are adjoining. However, to narrow the interval between the optical frequencies causes the following problems: (1) a problem of crosstalk between signals, especially of coherent crosstalk between signals caused by a spread spectrum upon a signal modulation, and (2) a problem of crosstalk between wavelengths caused by a non-linear effect of an optical fiber.
[1] Although various optical circuits for converting a lightwave into a linear polarization of a predetermined direction have been devised, there has been no simple optical circuit for converting a lightwave of an arbitrary polarization into a linear polarization of a predetermined direction only with an electric signal without a mechanical operation.
[2] On the other hand, as a method of overcoming the WDM problems, a transmission method of orthogonal polarization has been proposed. In this method, polarization directions of signal lights for odd even channels are orthogonalized to be transmitted. Namely, a transmitter sets the polarization directions of the optical signals for the odd even channels to be orthogonal and transmits the same. The orthogonal state is almost maintained in an optical transmission line.
In the WDM optical transmission apparatus 201 with n-channels shown in FIG. 8, the polarization states from the optical transmitter 30 to the coupler 35 are not managed, so that the polarization relationship between the channels is random.
The optical fibers 32 and 32xe2x80x2 of the WDM optical transmission apparatus 201 are connected with a polarization holding fiber, thereby enabling the orthogonal state to be held. However, there is a problem that the optical amplifier 31 composed of an erbium-doped polarization holding fiber has a low manufacturability and is expensive.
Also, in case of a WDM optical transmission apparatus 202 where the distance between the transmitters 30_1-30_n and the coupler 35 is long, the arrangement where the optical transmitters 30_1-30_n and the coupler 35 are respectively connected with the optical amplifiers 31_1-31_n, dispersion compensating fibers 33_1-33_n, and optical amplifiers 34_1-34_n is essential, and the dispersion must be compensated by the polarization holding fiber, so that there is a problem that the practicability of this arrangement is low.
Thus, in the prior art, there has been no WDM optical transmission apparatus having a method and an arrangement of a practical level on which polarizations of a plurality of modulated optical signals are managed, and odd even channels are mutually orthogonalized to be coupled.
It is accordingly an object of the present invention to provide a polarization compensator for converting a lightwave only a linear polarization or at least one of a linear polarization, a circular polarization, and an elliptical polarization into a linear polarization having a polarization plane of a predetermined direction, and a wavelength division multiplexing apparatus for coupling lightwaves so as to prevent polarization planes of the lightwaves whose wavelengths are adjoining from coinciding with each other by using the polarization compensator.
It is to be noted that the xe2x80x9cpolarization compensationxe2x80x9d in the specification of the present invention means that a lightwave of an arbitrary polarization is converted into a linear polarization having a polarization plane of a predetermined direction.
In order to achieve the above-mentioned object, a polarization compensator according to the present invention comprises: a rotator for rotating an input lightwave by an angle designated with a control signal and providing an output lightwave; a polarizer, having a polarization plane set in a reference direction, for inputting the output lightwave; and a controller for outputting the control signal which gives instructions to make a polarization plane of the input lightwave the reference direction based on a polarization signal from the polarizer (claim 1).
Namely, a rotator outputs an input lightwave rotated by an angle designated with a control signal from a controller as an output lightwave. As for a polarizer which inputs the output lightwave, a polarization plane is set in a reference direction. The controller gives instructions to the rotator with the control signal, based on a polarization signal from the polarizer, to make the polarization plane of the input lightwave the reference direction.
Thus, the polarization plane of the input lightwave is compensated to the output lightwave having the polarization plane of the reference direction.
Also, in the present invention according to the above-mentioned invention, the controller may comprise an oscillator for outputting a modulating signal; a polarization signal detector for converting the polarization signal from the polarizer into an electric signal; a phase comparator for determining whether the modulating signal and the electric signal are in phase or opposite phase (anti-phase), or whether or not a frequency of the electric signal is twice as high as a frequency of the modulating signal; a rotator controller for outputting the control signal which designates a rotation angle, based on a determination result of the phase comparator so that the frequency of the electric signal becomes twice as high as the frequency of the modulating signal only when the modulating signal and the electric signal are in phase or opposite phase; and a modulator for modulating the control signal with the modulating signal (claim 2).
Namely, an oscillator outputs a modulating signal. A polarization signal detector converts the polarization signal from the polarizer into an electric signal. A phase comparator determines whether the modulating signal and the electric signal are in phase or opposite phase, or whether or not a frequency of the electric signal is twice as high as that of the modulating signal. A rotator controller provides the control signal to the above-mentioned rotator, which designates a rotation angle based on a determination result of the phase comparator.
Thus, the controller can instruct the above-mentioned rotator to compensate the polarization plane of the input lightwave to the reference direction with the control signal.
Also, in the present invention according to the above-mentioned invention, a branching portion may further be comprised for branching an output lightwave from the rotator to be provided to the polarizer (claim 3).
Namely, a branching portion can branch an output lightwave of the rotator to be provided to the polarizer. Another branched output lightwave of the branching portion may be made the output lightwave of the entire polarization compensator.
Also, in the present invention according to the above-mentioned invention, a branching portion may further be comprised for branching an output lightwave from the polarizer to be provided to the polarization signal detector (claim 4).
Namely, a branching portion can branch an output lightwave from the polarizer to be provided to the polarization signal detector. The other branched output lightwave of the branching portion may be made the output lightwave of the entire polarization compensator.
Also, in the present invention according to the above-mentioned invention, when the rotator is named a first rotator, a second rotator may further be comprised for receiving a lightwave branched from the branching portion to be provided to the polarizer; and the controller may include an oscillator for outputting a modulating signal; a second rotator controller for controlling a rotation angle of the second rotator with the modulating signal; a polarization signal detector for converting the polarization signal from the polarizer into an electric signal; a phase comparator for determining whether the modulating signal and the electric signal are in phase or opposite phase, or whether or not a frequency of the electric signal is twice as high as a frequency of the modulating signal; and a first rotator controller for outputting the control signal which designates a rotation angle of the first rotator, based on a determination result of the phase comparator so that the frequency of the electric signal becomes twice as high as the frequency of the modulating signal only when the modulating signal and the electric signal are in phase or opposite phase (claim 5).
Namely, an oscillator outputs a modulating signal. A second rotator controller controls a rotation angle of the second rotator with the modulating signal. The lightwave outputted from the branching portion is inputted to a polarization signal detector through the second rotator and the polarizer to be converted into the electric signal at the polarization signal detector.
A phase comparator determines whether the modulating signal and the electric signal are in phase or opposite phase, or whether or not a frequency of the electric signal is twice as high as that of the modulating signal. Based on this determination result, a first rotator controller outputs the control signal for designating a rotation angle of the first rotator.
Thus, the first rotator can compensate the input lightwave to the output lightwave having the polarization plane of the reference direction designated by the polarization plane of the polarizer.
Also, in the present invention according to the above-mentioned invention, when the modulating signal and the electric signal are in phase or opposite phase, the phase comparator may include an amplitude of the electric signal in the determination result to be notified to the rotator controller, and the rotator controller may output the control signal based on the amplitude (claim 6).
Also, in the present invention according to the above-mentioned invention, when the polarizer is named a first polarizer, a second polarizer, having a polarization plane set in a direction different from the reference direction, for inputting a lightwave branched from the branching portion may further be comprised; and the controller may include a first polarization signal detector for converting the polarization signal from the first polarizer into a first electric signal; a second polarization signal detector for converting a polarization signal from the second polarizer into a second electric signal; and a rotator controller for outputting the control signal which designates a rotation angle of the rotator based on a difference between the first and the second electric signals (claim 7).
Namely, a first and a second polarizers whose polarization planes are different from each other are provided. The lightwaves branched from the branching portion are provided to the first and the second polarizers. A first and a second polarization signal detectors convert the polarization signals from the polarizers into a first and a second electric signals. A rotator controller outputs the control signal, based on the difference between the first and the second electric signals, which designates a rotation angle of the rotator.
Thus, the rotator can compensate the polarization plane of the input lightwave to the polarization plane of the reference direction.
Also, in order to achieve the above-mentioned object, the present invention may comprise: a rotator for rotating a polarization plane of an input lightwave by an angle designated with a control signal to provide an output lightwave; a first xc2xc wave plate for inputting the output lightwave of the rotator; a polarization compensator, according to claim 1, for inputting the output lightwave of the first xc2xc wave plate; a branching portion for branching the output lightwave of the polarization compensator; a second xc2xc wave plate for inputting the lightwave branched from the branching portion; a polarizer, having a polarization plane set in a reference direction, for inputting the output lightwave of the second xc2xc wave plate; and a controller for outputting the control signal which gives instructions to compensate the input lightwave to a linear polarization having the polarization plane of the reference direction based on a polarization signal from the polarizer (claim 8).
Namely, a rotator rotates the polarization plane of an input lightwave by an angle designated with a control signal to provide an output lightwave. A polarization compensator according to claim 1 inputs the output lightwave of the rotator through the first xc2xc wave plate.
A polarizer having a polarization plane set in a reference direction receives the output lightwave of the polarization compensator through the branching portion and the second xc2xc wave plate. A controller outputs the control signal which gives instructions to convert the input lightwave to a linear polarization having the polarization plane of the reference direction.
Thus, the input lightwave of a linear polarization, a circular polarization, or an elliptical polarization is compensated to a linear polarization having the polarization plane of the reference direction.
Also, in the present invention according to the above-mentioned invention, the controller may comprise an oscillator for outputting a modulating signal; a polarization signal detector for converting the polarization signal from the polarizer into an electric signal; a phase comparator for determining whether the modulating signal and the electric signal are in phase or opposite phase, or whether or not a frequency of the electric signal is twice as high as a frequency of the modulating signal; and a rotator controller for outputting the control signal which designates a rotation angle, based on a determination result of the phase comparator so that the frequency of the electric signal becomes twice as high as the frequency of the modulating signal only when the modulating signal and the electric signal are in phase or opposite phase (claim 9).
Namely, an oscillator outputs a modulating signal. A polarization signal detector converts the polarization signal from the above-mentioned polarizer into an electric signal. A phase comparator determines whether the modulating signal and the electric signal are in phase or opposite phase, or whether or not a frequency of the electric signal is twice as high as that of the modulating signal. Based on this determination result, a rotator controller provides the control signal which designates a rotation angle to the rotator.
Thus, the controller can perform control of compensating the input lightwave of the rotator to the linear polarization having the polarization plane of the reference direction.
Also, in the present invention according to the above-mentioned invention, when the rotator is named a second rotator, a third rotator inserted between the branching portion and the polarizer may further be comprised; and the controller may include an oscillator for outputting a modulating signal; a third rotator controller for controlling a rotation angle of the third rotator with the modulating signal; a polarization signal detector for converting the polarization signal from the polarizer into an electric signal; a phase comparator for determining whether the modulating signal and the electric signal are in phase or opposite phase, or whether or not a frequency of the electric signal is twice as high as a frequency of the modulating signal; and a second rotator controller for outputting the control signal which designates a rotation angle of the second rotator, based on a determination result of the phase comparator so that the frequency of the electric signal becomes twice as high as the frequency of the modulating signal only when the modulating signal and the electric signal are in phase or opposite phase (claim 10).
Namely, an oscillator outputs a modulating signal. A third rotator controller controls the rotation angle of the third rotator with the modulating signal. A polarization signal detector converts the lightwave, into an electric signal, from the above-mentioned branching portion received through the third rotator and the polarizer.
A phase comparator determines whether the modulating signal and the electric signal are in phase or opposite phase, or whether or not a frequency of the electric signal is twice as high as that of the modulating signal. Based on this determination result, a second rotator controller outputs the control signal which designates a rotation angle of the second rotator.
Thus, the controller can perform control of compensating the input lightwave of the rotator to a linear polarization having the polarization plane of the reference direction.
Also, in the present invention according to the above-mentioned invention, when the polarizer is named a first polarizer, a second polarizer, having a polarization plane set in a direction different from the reference direction, for inputting the lightwave from the xc2xc wave plate may further be comprised; and the controller may include a first polarization signal detector for converting the polarization signal from the first polarizer into a first electric signal; a second polarization signal detector for converting a polarization signal from the second polarizer into a second electric signal; and a rotator controller for outputting the control signal which designates a rotation angle of the rotator based on a difference between the first and the second electric signals (claim 11).
Also, in the present invention according to the above-mentioned invention, the controller may comprise only a light/electricity converter for converting the polarization signal into an electric signal; and an electric circuit for outputting the control signal based on the electric signal (claim 12).
Also, in the present invention, a wavelength division multiplexing apparatus may comprise a plurality of polarization compensators according to the above-mentioned invention; and a coupler for coupling a lightwave of a linear polarization from the polarization compensators; and the polarization compensators and the coupler may be connected so as to prevent polarization planes of adjoining lightwaves from coinciding with each other (claim 13).
Also, in the present invention according to the wavelength division multiplexing apparatus of the above-mentioned present invention, a junction, having a connecting mechanism which prevents the polarization planes of the adjoining lightwaves from coinciding with each other, may be provided between the polarization compensators and the coupler (claim 14).
Namely, by a mechanism of a junction, it becomes possible to prevent the polarization planes of the adjoining lightwaves from coinciding with each other.
Also, in the present invention according to the wavelength division multiplexing apparatus of the above-mentioned present invention, the polarization planes of the adjoining output lightwaves may be connected to be orthogonal (claim 15).
Also, in the present invention, a wavelength division multiplexing optical transmission apparatus may comprise: a plurality of optical transmitters whose output wavelengths are different from each other; and the above-mentioned wavelength division multiplexing apparatus for inputting the output lightwaves of the optical transmitters (claim 16).